1. Field of the Invention
The present invention relates to an image forming apparatus, such as a digital copying machine, which forms, by making use of an electrophotographic process, an image on a fed paper sheet.
2. Description of the Related Art
A conventional image forming apparatus using an electrophotographic process, for example, a digital copying machine, includes an image forming section which forms an image based on image data and outputs the image onto a recording medium such as paper.
The image forming apparatus has a photosensitive drum that carries thereon an electrostatic latent image corresponding to the image data. The image forming apparatus includes a charger for uniformly charging the surface of the photosensitive drum to a predetermined potential; a developing device for supplying a developer, i.e., a toner, to the electrostatic latent image that is formed by exposing the charged surface of the photosensitive drum, thus forming a toner image; a transfer belt for transferring the toner image from the photosensitive drum onto a recording medium such as a paper sheet; a drum cleaner for removing residual toner on the photosensitive drum after the transfer of the toner image; and a charge erase lamp for erasing the charge remaining on the photosensitive drum. These components are arranged around the photosensitive drum in the named order in the rotational direction of the photosensitive drum.
In a high-humidity environment, however, there arises such a problem that when toner is supplied to the developer, the rise in charge becomes slower than in a normal-humidity or low-humidity environment due to moisture-absorption of the toner (developer) itself. If image formation is performed in the state in which the supplied toner is not sufficiently charged, image fogging of the toner occurs. The image fogging, in this context, refers to a phenomenon where toner forms on an area that should normally become a white area in the image formation, and this area becomes gray.
In particular, in the case of recycled toner (recovered toner) once used for image formation, an external additive for controlling the charge amount has already been removed from the toner, so the phenomenon of image fogging becomes conspicuous, compared to fresh toner (new toner).
In a forcible toner replenishment mode that is performed for restoration from a toner empty state, a great deal of toner is supplied in a shorter time than in the normal mode, thus aggravating the phenomenon. To cope with the problem, there is a publicly known method in which replenishment of recycled toner is prohibited during this time, and only fresh toner is supplied.
In such a case, even when the forcible toner replenishment mode is finished and the normal mode is restored, the charge amount (level) of the developer is still lower than in the normal state and the replenishment of recycled toner results in frequent occurrence of image fogging.